There are basically two forms of life on earth: prokaryotes, which are cells without a nucleus, and eukarotes, which are cells which contain a nucleus (Barbieri, 2001). Prokaryotes are found almost exclusively as single-celled organisms which can inhabit any ecological niche. They can exist with or without light, oxygen or organic molecules. They synthesize all their components from inorganic molecules, can rapidly adapt to environmental changes, and can exchange genes horizontally. Prokaryotes have a single circular DNA molecule, a single cell compartment and an exoskeleton.
Eukaryotes are mostly dependent on oxygen, and exists as monocellular organisms such as protozoa and protista, and have given rise to the three kingdoms of multicellular organisms that exist: plants, fungi and animals (Barbieri, 2001). They have developed new types of cell division (mitosis and meiosis), new types of movement, meiotic sexuality and embryonic development. The latter process is capable of generating endless different structures. Eukaryotes have DNA molecules arranged in linear fibers, and their cytoplasm contains several compartments (mitochondria, chloroplasts, lysosomes, etc.). They can adapt to environmental changes by changing anatomically, but at the price of extinction of many forms over time. The average life of a eukaryote type is four million years, whereas some bacterial prokaryotes have been estimated to be three billion years old.
It has been proposed that the first forms of life had no nucleus, but gave rise to nucleated cells by engulfing bacteria in a symbiotic relationship. These bacteria became chloroplasts and mitochondria (Barbieri, 2001). These two intracellular organelles contain fragments of ancient circular DNA, and have 70s ribosomes, which confirms their bacterial origins. It has been proposed that symbiosis of eukaryotes and bacteria required characteristics which bacteria do not possess, and so there...